Automated mail sorting machines such as is disclosed in U.S. patent application Ser. No. 07/961,980 filed Oct. 6, 1993, the disclosure of which is hereby incorporated by reference, and assigned to the U.S. Postal Service depends upon a reliable singulator. In that automated machine, a stack of mail is initially fed into a shingler which produces a shingled stream of mail. The singulator then separates each piece and orients it on a conveyor with a predetermined distance between the trailing edge of a leading piece and the leading edge of the next trailing piece. Downstream of the singulator then, scanners are provided which scan, for example, a bar code thereon and, on the basis of that scan, the microprocessor sorts the mail or other documents. In the above-identified mailing machine, typically 3,000 pieces of mail are subjected to three passes and thereby may be sorted by address and doorway for an individual mail carrier's route, in a matter of minutes.
Singulators are also used for the controlled input of machinable letter mail to most automated mail processing equipment including facer-cancellers, optical character readers, bar code readers, labelling machines and mail sorting equipment as described above. The singulator in general accepts either a stack of edged letters and postcards from a feed table or a continuous shingled stream of letters and postcards from an upstream source and separates the front or foremost mail piece by feeding it laterally while retarding the remaining mail pieces. This action produces a gap between the trailing edge of the first mail piece and the leading edge of the next trailing mail piece to be singulated. The individual letters and postcards can then be scanned, cancelled, or the like, depending upon the function of the machine.
There are many different types of letter or document singulators in use for commercial and U.S. Postal Service mail processing operations. Such machines usually are classified as synchronous or asynchronous and vacuum or friction types. A synchronous singulator feeds each mail piece from a stack or shingled stream with the leading edges spaced a constant (fixed) pitch as they move along the downstream transport. Asynchronous singulators feed each mail piece with a constant gap between the trailing edge of the upstream letter and the leading edge of the downstream letter and, therefore, they are fixed gap singulators. Vacuum singulators are characterized by the use of a negative pressure to separate the first mail piece from each stack. The vacuum can be applied by means of vacuum cups attached to a reciprocating arm or a perforated belt or belts moving across a vacuum manifold. Friction singulators use either friction wheels or belts that are located at the front of the mail stack or stream and apply the separating force to the first mail piece.
Since the separating force on the front mail piece usually tends to carry more than one mail piece at a time from the stack, a stripper mechanism is required at the exit point from the stack to retard all letters except the first one. The stripper can be a wiper, several small, spring loaded friction pads or a more positive counter rotating stripper wheel or belt. In each case, the stripper mechanism must apply sufficient force on the letter or letters to stop their forward motion while not restricting the forward motion of the first letter.
After initiating the singulation function with the vacuum or friction wheel or belt, the first mail piece reaches a zone located a predetermined distance from its starting point where it must be accelerated to the output transport. An adjacent pair of friction covered pulleys or pinch belts provide the accelerating force for each letter.